The present invention relates to a polishing pad used in manufacturing magnetic disks for magnetic recording media in information recording devices such as hard disks, magneto-optic disks, and optical disks, a method of manufacturing glass substrates for use in data recording media by using the pad, and a glass substrate for use in data recording media obtained by the method.
Conventionally, magnetic disks, which are one type of data recording media, are used in hard disk drive devices. Such magnetic disks are manufactured by laminating magnetic layers on the surface of glass substrates for use in data recording media (hereinafter referred to as glass substrates). In this connection, a magnetic head for reading information recorded on a magnetic disk (hereinafter sometimes referred to as a head) moves relative to the magnetic disk in a manner floating above and away from the surface of the magnetic disk.
When the head moves, if there are asperities on the surface of the magnetic disk, the head comes into contact with the asperities, so that there possibly occurs a problem in that the head is damaged and/or the magnetic disk scratched. Additionally, current magnetic disks are required to have increased memory capacity, and in order to meet this requirement, it is necessary to enhance the recording density by narrowing to a possible extreme the spacing between the surface of the magnetic disk and the head. Accordingly, glass substrates for magnetic disks are subjected to polishing treatment, and in this connection, an attempt has been made for improving the surface flatness for the purpose of suppressing the generation of surface asperities (for example, see Japanese Laid-Open Patent Publication No. 2000-288922).
In the above described polishing treatment, a polishing apparatus equipped with a polishing carrier and a polishing pad is used; when performing polishing treatment, the polishing pad is brought into contact with the surface of the glass substrate under the condition such that the glass substrate is accommodated on a polishing carrier, the glass substrate and the polishing pad are made to mutually rotate, and thus the surface of the glass substrate is polished.
Now, when a glass substrate manufactured by use of the above described polishing pad fails to meet the required qualities, the pad is regarded to have reached its durability limit and accordingly replaced with a new polishing pad. Conventionally, there has been selected a polishing pad having hardness in compliance with the purposes involving the carrier hardness, glass substrate hardness and the like, by referring to the Asker C hardness specified in the standard SRIS-0101 of the Society of Rubber Industry, Japan. According to the selected pad, the quality required for the glass substrate and the replacement timing of the polishing pad are determined. The polishing pad is composed of a base made of nonwoven fabric and a polishing portion made of synthetic resin foam and laminated on the surface of the base for contact with the surface of a glass substrate. However, the Asker C hardness is the hardness obtained by measurement of the polishing pad, after the base and the polishing portion have been laminated together, but is not the hardness obtained by measurement only of the polishing portion that is actually for contact with the surface of a glass workpiece.
More specifically, the polishing pad selected on the basis of the Asker C hardness does not involve consideration of the time variation of the polishing portion thereof. Thus, according to the polishing pad selected on the basis of the Asker C hardness, the time variation of the polishing portion leads to a fear of causing a problem including the possibility such that the quality of the manufactured glass substrate is not stabilized and the polishing pad must be frequently replaced. Because of a fear of causing such a problem, there has been a problem such that it is difficult to increase the manufacturing quantity while maintaining the process yield of the glass substrate with stable quality.